POLYCHAETA. 459 



but in a large number of cases partly or entirely break down and 

 disappear. As a rule there are no special generative ducts, and the 

 generative products, which are dehisced into the coelom, pass out by 

 any or some of the nephridia. A vascular system is present, except 

 in a few cases, in which its absence, though stated on good authority, 

 must be regarded as doubtful until renewed investigations of the 

 matter have been made. Some are free, but a large number inhabit 

 tubes which they manufacture or construct for themselves. 



The body is generally elongated, and the internal segmentation 

 shown externally. The number of segments is usually considerable, 

 and in some cases is variable, increasing with age. The segments 

 may be all alike, or the body may be divided into two regions, often 

 called thorax and abdomen, which differ from one another in the 

 form of the parapodia, of the setae, and in other respects (Sabellidae, 

 Capitellidae, etc.). In some forms the hind end of the body is much 

 reduced, and may be without setae. 



The head consists of the anterior part of the body, on the 

 ventral side of which the mouth is placed. It is almost always 

 divided by a mark into a preoral portion the prestomium, and 

 a postoral portion, which is miscalled the peristomium. The pres- 

 tomium is often called the first segment, and the peristomium the 

 second; but in this work the peristomium is reckoned as the first. 



The prestomium may be a well-marked structure, or it may be 

 much reduced, and hardly distinguishable from the peristomium 

 (Arenicola). In the Cryptocephala the prestomium is hidden by 

 the forward extension of the peristomium. The prestomium may 

 be without appendages, but it usually bears two kinds of sensory 

 appendages the tentacles, which are attached dorsally and vary in 

 number from two to five, and the palps which are two in number 

 and ventrally placed (Fig. 363). It often bears one or two pairs 

 of eyes. The peristomium is, in rare cases, provided with parapodia 

 and setae, like the other segments (Aphrodite, Hermione, Nephthys) ; 

 usually it is without setae and parapodia, though the cirri are often 

 present as long, tentacle-like structures called tentacular cirri (Fig. 

 363, Fc). When there is more than one pair of tentacular cirri it 

 is supposed that one or more of the hinder segments have become 

 merged in the peristomium, and have lost their parapodia; in such 

 cases the head is a compound or secondary head, one or two body 

 segments having fused with the peristomeal. 



The parapodia are segmental, hollow, lateral projections of the 

 body. They carry the setae and are either biramous or uniramous. 



